Unmanned aerial vehicles (UAV) are types of aircraft that fly with no pilot, crew, or passengers onboard. UAVs include both autonomous and remotely piloted vehicles. The earliest UAVs were balloons loaded with explosives, which were used by the Austrians in the mid-1800s. Shortly after World War I radio controlled aircraft were developed. Later developments in this technology led to more sophisticated UAVs which included remotely controlled UAVs as well as autonomously guided vehicles. As UAVs become more sophisticated, they may include more electronics, motors, and other heat sources. UAVs may also be operated for longer durations than their predecessors.